1. Field of the Invention
This invention relates to an interferometric apparatus and process for determining and measuring defects, by means of the interference of beams modulated and not modulated at the object. The invention further relates to an improved interferometric apparatus and process for the positioning of objects showing periodically structurized surfaces.
2. Description of the Prior Art
In many technical fields, for instance in the production and testing of the imaging quality of high resolution optical systems, or in the fabrication and examination of gratings and grating-like structures it is necessary to detect and measure the presence and magnitude of irregularities or disturbances of the periodicity of objects with periodically structurized surfaces. The transmission properties of high resolution imaging systems used in the manufacture of integrated circuits must be checked for each setting. The best way to do this is to check and measure a micro-grating that is made with the system to be examined.
In the fabrication of semiconductor integrated circuits, a semiconductor wafer having a surface area of approximately 5 cm.sup.2 generally has thousands of circuits thereon. Frequently, as in the case of bistable storage circuits, many of these circuits are identical to each other. Also, the masks used for making the integrated circuits consist of periodical structures with grating constants of 1 to 5 .mu.m. Deviations from the periodicity in general cause disturbances of the electric properties of the integrated circuits made thereby. The same applies also to the integrated circuits themselves.
Disturbances of the periodicity of the transmitted patterns can also occur when using so-called "step-and-repeat" cameras where it is well known that the light pattern transmitted onto a photoresist-coated semiconductor wafer is produced by transmitting hundreds or even thousands of precisely aligned partial patterns by means of a corresponding number of individual exposures.
The importance of maintaining an optimum periodicity and a maximum positional precision of the transmitted patterns is evident, inter alia, from the fact that in the production of integrated circuits, during 20 to 30 successive production steps, 20 to 30 precisely aligned and partly overlapping light patterns are applied. The line widths and spacings of said light patterns are in the order of a few .mu.m.
In view of the trend towards integrated circuits with decreasing dimensions of the structural details and increasing packing densities, requirements for exact dimensional stability and periodicity of the transmitted patterns, as well as measurements permitting a checking of these characteristics are increasingly important.
In the known prior art, these measurements could be performed only by means of comparative measurings, for instance between two masks or between a mask and a semiconductor wafer. Since this kind of measuring involved specific test masks and a multitude of measurement data, as well as their correlation with the respective measurement data of the reference objects these processes, apart from the high amount of apparatus and costs they involved, could not be applied for the continuous checking of the production.
German Offenlegungsschrift No. 2,150,110 describes a process for determining the displacement or deformation of a periodically structurized object, where two beams derived from a common beam are applied onto the object in such a manner that two different diffraction orders differing from the zero'th order, of the two wavefronts are combined and brought into interference by an imaging element. Since according to this process both beams are influenced by the structure to be examined there is, apart from a strongly reduced sensitivity, also the disadvantage that disturbances appearing in the interference field cannot be correlated with specific locations of the objects which are being examined. This process involves not only an increased amount of apparatus but also additional inaccuracies which are due to the imaging errors of the imaging systems used.